Stereoscopic type optical rangefinders derive target range by measuring the angle of parallax subtended by the target of interest and the left and right hand entrance windows whose spacing determines the baselength of the rangefinder. Stereoscopic rangefinders typically depend upon the human users perception of depth to compare the depth of a real target scene with that of an artificial reference image. Provision is made for the user to adjust the angle of view of either the real target scene, or the artificial reference image so that their apparent depth is the same. When this adjustment is made, the observed effect is that of the artificial reference image appearing to rush up to, or alongside the target scene. The position of the adjuster may then be translated to the angle of parallax, or range to the target.
Stereoscopic rangefinders differ fundamentally from coincidence type rangefinders in that the former measures the viewing angle of images between separated optical channels, while the latter seeks to measure the alignment of dual or split images merged into a single optical channel. As such, coincidence type optical rangefinders generally employ a central beamsplitter to merge two beams of light from separated entrance windows whereas stereoscopic rangefinders generally lack this beamsplitter element. Also of importance is that although most persons are readily capable of aligning dual or split images presented to the eye, stereoscopic rangefinders are not used effectively by all persons and generally training and familiarity are required before proficiency is attained. This is the main reason few stereoscopic rangefinders are publicly available while numerous types and brands of coincidence type rangefinders exist.
Optical rangefinders, and in particular, stereoscopic rangefinders have suffered from many shortcomings some of which follow:
1. Accuracy of ranging is dependent upon the users ability which can vary widely from one user to the next.
2. Existing devices are generally sensitive to slight movement of optical components caused by mechanical shock, or thermal influences which can result in loss of range accuracy.
3. Devices which are less sensitive to movement of optical elements generally substitute mirror reflectors with larger and heavier prisms.
4. Devices which employ a light deviating element to adjust the angle of view of either the real target scene or the artificial reference image also use mechanisms to translate this movement to a scale from which range may be read. These mechanisms are costly and are susceptible to wear and deformation of components which can also result in range inaccuracies.
5. Accuracy generally depends on the user's depth perception and particularly on his ability to judge relative distances. This ability may vary significantly from one person to the next.
U.S. Pat. No. 3,180,208 entitled "Optical Range Finding Device", to Swartz and Marasco (1965) for example, teaches a stereoscopic rangefinder where the operator alters the viewing angle of the real target scene by adjusting a light deviator, or compensator. In this example, the viewing angle of the artificial reference image, or stereo reticle pattern is fixed although in practice, the appearance is that the artificial reference image moves to meet the target. Although care has been taken in the design of the compensator and associated mechanism which reads the position of the compensator using anti-backlash gears, many moving parts are involved. This mechanism, apart from being costly, cannot avoid introducing range inaccuracies from wear, and mechanical or thermal deformations.
Another example of a stereoscopic rangefinder configured as a binocular is U.S. Pat. No. 4,886,347 entitled "Range-Finding Binocular", to Monroe, (1989). Different from Swartz and Marasco, this rangefinder provides for a fixed viewing angle to the target, and an adjustable viewing angle of the artificial reference image, in this case, an illuminated "range-mark". Adjustment of the range-mark is made by a fine gun screw driven by a reversible DC motor which also includes a position sensor whereby the position of the range-mark may be determined resulting in the apparent range of the real target. This system suffers from the problems associated with the use of mechanical components and with the problem of shifting optical elements by mechanical shock or thermal deformations which are otherwise indistinguishable from changes in the actual parallax angle of the target of interest. In other words, unlike the Swartz invention where the artificial reference images enter the visual optical system with a fixed parallax angle, Monroe's invention has no fixed reference and must therefore rely on very high optical and mechanical stabilities.
Still another example of a stereoscopic rangefinder also configured as a binocular is U.S. Pat. No. 4,071,772 to Leitz et al. (1978), entitled, "Apparatus for Measurement of Mechanical Aberrations Affecting Stereoscopic Image Analysis". This invention uses a spatial referencing system for producing laterally stable marker beams, and a common oscillating grating structure and drive means. As the grating structure is oscillated back and forth in the reference mark and target images, light intensities registered by photodetectors will vary in time with the motion of the structure.
In the device of the present invention a fundamentally different system is used to directly measure target image, and reference mark image separations, independent of any oscillating measurement structure, or optical grating. In the device of the present invention, no moving elements are involved in the calculation of target range which would otherwise be costly and subject to friction and wear. U.S. Pat. No. 4,465,366 entitled, Device for the Photo-electric Determination of the Position of at Least One Focal Plane of an Image, to Schmidt (1984), also describes a ranging system using an oscillating grating structure similar to the cited Leitz invention.
A final example of another stereoscopic rangefinder is described in an article prepared by the Optics Research Department of British Aerospace entitled, "Applied Research Project Passive Stereoscopic Rangefinder", (1983) This article describes a stereoscopic rangefinder using dual photodetectors, and image correlation and processing techniques to measure a target angle of parallax. Mechanical and thermal stability is achieved in part through the use of a single common objective lens through which first and second channel target beams pass. It is assumed that the use of a single objective lens will overcome the problem of objective lens movement. Stability is also achieved by the use of diffraction alignment systems which use the principle of Young's Fringes. One of such systems may be placed at one location in the optical system to record and by some means, (not described), correct for movement of beamsplitter, mirror and photodetectors, and at another location, to correct for movement of the two primary mirrors. No explanation is offered how interference patterns produced by such an alignment system on the photodetectors may be analyzed, and more importantly, separated from the target scene images. Also, although in this particular system, the use of a single objective lens may overcome the problem of lens movement, it requires that the lens be centralized between the first and second optical channels. To accommodate any reasonable field of view in such a system, the primary mirrors and entrance windows would be necessarily large which may be impractical, at least for use within a hand-held binocular.
More to the point is that in the device of the present invention, a fundamentally different system is used to correct for movements of elements of the ranging system including objective lenses, and photodetectors. This system is direct, and does not require the analysis of diffraction patterns produced by diffraction alignment systems introduced at different locations in the optical system. Further, the device of the present invention clearly provides means for extracting, and using information from the photodetectors to correct for movements of elements involved in the measurement of range.